Ikeda et al. (Non-Patent Document 1) indicates that when an “AlTiN” or “AlTiCN” coating with an atomic ratio of Al exceeding 0.7 is produced by physical vapor deposition (PVD), the layer structure of the coating changes to the wurtzite crystal structure, resulting in reduction of hardness. Setoyama et al. (Non-Patent Document 2) produced a super multilayer film of TiN/AlN by PVD in order to increase an Al content in an “AlTiN” or “AlTiCN” coating. However, it is reported that when an “AlTiN” or “AlTiCN” coating with a thickness exceeding 3 nm per AlN layer is produced, the layer structure changes to the wurtzite crystal structure, resulting in reduction of hardness. Studies have been made for further improvement in cutting performance, for example, by techniques such as increasing the atomic ratio of Al by chemical vapor deposition (CVD).
For example, Japanese Patent Laying-Open No. 2014-129562 (Patent Document 1) discloses a method in which AlCl3 gas, TiCl4 gas, NH3 gas, H2 gas, and N2 gas are introduced into a reaction chamber at a pressure of 1.3 kPa and a temperature of 800° C. and then the reaction chamber is cooled at a cooling rate of 10° C./min until the temperature of the base material reaches 200° C. It is said that, with this method, a hard coating having a structure in which a 2 nm-thick TiN having the face-centered cubic lattice (fcc) structure and a 6 nm-thick AlN having the fcc structure are alternately layered can be formed by CVD.
Furthermore, Japanese Patent Laying-Open No. 2015-193071 (Patent Document 2) discloses a hard coating layer including a composite nitride layer or a composite carbonitride layer represented by (Ti1-xAlxN)(CyN1-y), in which the layer includes crystal grains having a cubic structure, and the composition of Ti and Al periodically varies along the normal direction to the surface of the tool base material.